A Boule de Moulins was a method of transporting mail from the provinces to the city of Paris, used during the siege of the 1870 war. As the mail to be transported in this way in hollow spheres is first centralized in Moulins (Allier), these balls are called "de Moulins". Three people came up with the idea of placing the mail in watertight zinc cylinders, given a spherical shape by means of fins, which were immersed in the Seine well upstream of Paris, not far from Montereau-Fault-Yonne. Rolling downstream on the river bed, these balls were to pass through the lines of the besiegers and be collected in a net set up on the outskirts of the capital. However, operations were delayed by lengthy discussions with the Post administration, as winter advanced and ice encumbered the course of the Seine. Despite promising trials, the process proved a complete failure, with none of the 55 Moulins balls dropped between January 5 and 28, 1871 reaching Paris before the end of the siege, although more than half of them have since been found, from the Seine-et-Marne to the mouth of the Seine. The letters they contained were handed over to their addressees or descendants, or found their way into the collections of the Musée de La Poste and private collections.
Maintaining contact between Paris and the provinces
Continued correspondence between the provinces and the capital, which had been under siege by the Prussian army since September 19, 1870, was of the utmost importance. The exchange of information on the military situation was vital; soldiers on the battlefields needed the moral support of their families back in Paris, to whom they wanted to give news in return; the besieged, for their part, needed to continue to feel connected to the provinces. Various means were used to try and maintain this relational bridge through the exchange of mail, but mainly in the Paris-province direction, such as the balloon mail, which could carry several bags of mail; for the duration of the siege, 67 of these balloons were produced in the Gare du Nord, Gare de l'Est and Gare d'Orléans, which were free of rail traffic at the time. In the other direction, the problem was more complex. The air route was difficult to use: only carrier pigeons managed to enter the capital, but their numbers were insufficient for the volume to be transported, and the failure rate was high. Roads were blocked and the gates of Paris guarded: messengers attempting to break the blockade were shot as spies after an expeditious military trial, and dogs trained to enter the city were shot on sight. Trains stopped running, and the Prussians watched and patrolled the Seine, preventing the passage of anything floating on the river's surface.
Principle, description and tests
As early as September 1870, Mr. Castillon de Saint-Victor was considering the idea of using the Seine's current to carry watertight balls containing mail, based on the "message in a bottle" principle. He proposed devices that would float below the surface, releasing a small flag once they entered Paris to signal their presence. The project was quickly abandoned as too complicated and risky to implement.
However, the idea was explored again at the beginning of the following month, but to thwart the besiegers' surveillance, the balls must not float on the surface or between two bodies of water. Their density was calculated so that they would roll along the bottom, carried along by the current. Three people, Pierre-Charles Delort, Émile (or Louis-Émile) Robert and Isca (or Jacques) Vonoven, developed a system based on the manufacture of zinc cylinders, a metal unaffected by water, which were watertight once the lid had been welded on. The device conceived by the three men, who did not seem to have been aware of Castillon de Saint-Victor's project, was perhaps inspired by the one used for tobacco smuggling on the French-Belgian border, in the waters of the Yser. The hollow cylinder was fitted with twelve turbine blade-like vanes designed to rotate it in the river current. With a total mass of just over 2 kg, the whole unit was shaped like a sphere and could hold between 500 and 600 letters, each weighing no more than 4 grams, including the envelope, written on peel-off paper. The cylinders had a small ballast at either end, which could be filled at will with water to balance the unit according to the weight of the letters it contained. The patent was filed by Delort on November 21, 1870. Two successive experiments, carried out over short distances in the Bièvre at Arcueil, followed by another on December 1 and 2, 1870 over a longer distance at Port à l'Anglais in the Seine, were conclusive: balls thrown into the water were caught in the nets designed to stop them; in the case of the Bièvre, the clarity of the water even made it possible to follow them on their way. The principle therefore seemed applicable on a much larger scale.
Implementation
Bargaining and influence struggles
First contract, with Germain Rampont
Robert met General Trochu in early October 1870 (they had met in Algeria twenty years earlier); Trochu showed great interest, and Robert immediately submitted the project to Germain Rampont, General Manager of the Post, who had remained in Paris. Once the tests had been carried out, Rampont drew up a contract on December 6, authorizing the implementation of this mail delivery service. Although the document was financially very interesting for the designers, it was vague on many practical details: for example, the collection point, Clermont-Ferrand, was only agreed verbally, and although the amount of the remuneration for the operators was fixed, the payment terms were not specified. Delort and Robert left the capital the following night aboard the Denis Papin balloon mail, which took off from the esplanade of the Gare d'Orléans. They took mail bond to the provinces with them, a few balls made in Paris and the contract signed by Rampont and approved by the Minister of Finance, Ernest Picard.
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